The Taxonomy of the Green Macroalgal Genus Caulerpa: A Framework for the Application of Old Species Names in DNA-Based Taxonomies

Abstract

Phenotypic plasticity has been a long-standing source of taxonomic difficulty in many algal groups. Nowhere is this more evident than in the siphonous green algal genera, Caulerpa, which although consisting of a single cell, display a complex habit with the thallus differentiating into stolons, rhizoids, and upright fronds, many also bearing branchlets. Environmentally controlled phenotypic plasticity in all morphological characters has led to much confusion, misidentification and a cumbersome classification scheme. Furthermore, recent molecular studies have added to this confusion by uncovering considerable conflict between morphological and molecular species concepts. A framework to resolve the taxonomy and match previously described names with newly obtained molecular data has remained an elusive goal to date for this group. The present thesis aimed to resolve the taxonomy of Caulerpa species found in Australian waters and in so doing, provide a best practice framework for integrating historical nomenclature into molecular taxonomies. Using a combination of phylogenetic and species-delimitation approaches of partial tufA and rbcL gene sequences, I first determined the number of genetic species within a globally sampled set of “C. racemosa-peltata complex” specimens. Guided by these results, comparative morphological examinations were then undertaken to gauge the extent of phenotypic plasticity within each species, as well as any morphological overlap between them. This approach was combined with detailed observations of type and herbarium specimens, and consideration of type descriptions to make informed decisions to advise on taxonomic updates. Using this framework I then tackled the taxonomies of the species rich Caulerpa floras of north-western and southern Australia. Unsurprisingly, results confirmed some serious mismatches between morphological and molecular species concepts in the genus, most evident in the C. racemosa-peltata complex. I describe a number of new species and make a large number of taxonomic changes. A number of range extensions for some species were also uncovered, most importantly, the recent southerly extension of C. cylindracea into Victorian waters (previously C. racemosa var. cylindracea), one of the most notorious invasive seaweeds in the world. With an accurate DNA barcode for C. cylindracea, I was then able to use Genbank records and newly obtained data in order to obtain an accurate global distribution for the species. Using this dataset species distribution modelling was used to predict future spread, and identified areas at risk of invasion along the coasts of Western Europe, western Africa and the south coast of Australia.